1. Field of the Invention
The present invention relates to a thermoelectric semiconductor material having thermoelectric conversion ability.
2. Description of the Related Art
In thermoelectric semiconductor material, a p-type or n-type material is connected directly or by way of a metal electrode to be a thermoelectric element. Then, heat energy is directly converted into electric energy (Seebeck effect). Or, an electric current is carried to adsorb and radiate the heat (Peltier effect). This element is used as a thermoelectric conversion device for thermoelectric cooling or thermoelectric generation or a variety of sensor using thermoelectromotive force.
An effective maximum power P.sub.max of the thermoelectric conversion element is represented as the following formula: EQU P.sub.max =1/4.times.S.sup.2 .DELTA.T.sup.2 /.rho. EQU =/14.times.E.sup.2 /.rho.
In this formula, S is Seebeck coefficient, .rho. is an effective mean resistivity of the thermoelectric semiconductor material, .DELTA.T is a temperature difference between a high electrode and a low electrode and E is thermoelectromotive force between a high electrode and a low electrode at the temperature difference .DELTA.T.
Generally, in a thermoelectric conversion element, in order to obtain high conversion efficiency (high effective maximum power), the value of Seebeck coefficient S (.mu.V/.degree.C) is set to be large. And, the effective mean resistivity .rho. and heat conductivity .kappa. is set to be small.
It is known that thermoelectric semiconductor material is metal silicide, typically, iron silicide.
The semiconducting iron disilicide (.beta.-FeSi.sub.2) is changed to a p-type material by adding a small amount of Mn or Al. Furthermore the .beta.-FeSi.sub.2 is changed to an n-type material by adding a small amount of Co or Ni.
It is known that the following is effective in order to decrease resistance without deteriorating thermoelectromotive force of the above-mentioned materials, and to improve generating efficiency. In "Abstracts of Japan Ceramics Association 1992, 2A36 (p.162) and 2A37 (p.163)", it is disclosed that Co, Mn and Cr are distributed in a grain boundary of iron silicide. In Japanese Unexamined Patent Publication (KOKAI) No. 169283/1982, it is disclosed that the thermoelectric semiconductor material has a compound structure comprising a metal phase (.gamma. phase) of iron silicide and a semiconductor phase (.beta. phase).
It is known that a special thermoelectric semiconductor material is FeSiO system material. An ionized cluster beam (ICB) of an oxygen gas is irradiated to iron silicide to make an aggregate comprising iron silicide and amorphous silicon dioxide, thereby improving a thermoelectromotive force. It is disclosed in Japanese Unexamined Patent Publication No. 35071/1992. However, this material is made only as a thin film by ICB or a deposition method. Furthermore, this material has poor reproducibility and poor durability. Therefore, it is difficult to use in practice.